The present invention relates to an apparatus for compression molding recorded discs which includes means for forming a center hole in the disc as the disc is being molded. More particularly, the present invention relates to such an apparatus which provides for greater ease of forming the hole in the disc.
A high information density recorded disc, such as a video disc, has a packing density of the information track in the order of 10,000 rotations per inch (3,937 per cm) so that the information track must be much finer in dimension than that of an audio disc, and rotates at a higher speed, about 450 r.p.m. during playback, as compared to 33 r.p.m. for an audio disc. Because of the fine dimensions of the information track and the high speed rotation of the disc, the disc must be stably supported to insure proper tracking of the stylus during playback. To help achieve this stability, the disc is provided with a relatively large center hole, about 1.3 inches (3.3 cm) in diameter for seating in the player on a truncated conical spindle.
A method generally used to make the disc is compression molding. It has been found desirable to form the center hole in the disc as it is being molded. An apparatus suitable for forming the center hole in the disc as it is being molded is shown and described in copending applications for U.S. Letters Patent of Michael Lee McNeely, Ser. No. 93,012, filed Nov. 9, 1979, entitled Method and Apparatus for Producing Disc Records having Molded-in Center Holes, Robert Winfield Chambers et al, Ser. No. 187,161, filed Sept. 15, 1980, entitled Apparatus for Molding a Recorded Disc, and John J. Prusak, Ser. No. 310,856, filed Oct. 13, 1981, entitled Apparatus for Compression Molding Recorded Discs, which are incorporated herein by reference.
The apparatus of each of the above applications includes a pair of mold plates, a center hole forming pin slidably mounted in one of the mold plates and a center plate in the other mold plate. The center hole forming pin and the center plate have surfaces which mate when forming the hole in the disc. The center hole forming pin has a recess in its mating surface. A disc is molded in this apparatus by placing a preform of plastic material between the mold plates, which are heated. The heated mold plates are brought together against the preform to heat the preform and then compress it so that the plastic material flows radially outwardly to fill the mold cavity formed between the mold plates. As the plastic material is being compressed, the center hole forming pin is moved through the plastic material toward the center plate in the other mold plate. As the center hole forming pin moves through the plastic material, it pushes the plastic material aside to form the hole. However, some of the plastic material fills the recess in the end of the pin and remains therein when the end surface of the pin mates with the opposed surface of the center plate. The center hole forming pin remains in contact with the center plate until the mold plates move completely together to form the disc and the mold plates are cooled to harden the disc. When the plastic material is cooled, the material that is in the recess in the end of the center hole forming pin is also hardened and is retained in the recess.
A problem which has arisen with regard to the above described apparatus is that it takes a relatively high pressure, about 6,000 pounds, to force the center hole forming pin through the plastic material. This high pressure not only can have an adverse affect on the quality of the hole formed in the disc, but also causes considerable wear on the pin itself. Since a present production mold press makes one disc about every 40 seconds, the repeated application of this high pressure to the pin over a single day's operation can cause considerable mechanical fatigue to the pin which could damage the pin.